The present invention relates to ink compositions, and particularly, to ink compositions containing one or more metal oxides which can improve various properties.
Inkjet printing is a non-impact process wherein droplets of ink are produced and deposited on a substrate such as paper, transparent film, or textile material in response to an electronic signal. Typical inkjet printing systems are continuous stream or drop-on-demand type. In continuous inkjet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed causing it to break up into droplets at a fixed distance from the orifice. At the breakup point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field. The field adjusts the trajectory of each droplet to direct it to a gutter for recirculation or to a specific location on a recording medium to create images. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium by pressure generated in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium. The drop-on-demand system has several advantages over the continuous system in that it requires no ink recovery, charging, or deflection.
Generally, there are three basic types of drop-on-demand inkjet systems. The first type is known as a piezoelectric system. In one kind of piezoelectric printer, ink fills a channel with a nozzle on one end and a piezoelectric transducer produces pressure pulses near the other end.
A second type of drop-on-demand system is known as thermal inkjet or bubble jet. The major components are an ink-filled channel with a nozzle at one end and a heat generating resistor near the nozzle. Printing signals create an electric current pulse in a resistive layer within each ink passageway, causing the ink in the immediate vicinity to vaporize, creating a bubble. Some of the ink in the channel is forced out through the orifice as a propelled droplet due to bubble expansion. Thermal or bubble inkjet printers produce high velocity droplets and allow very close spacing of nozzles, which results in a high quality of print.
The third type of drop-on-demand inkjet device is known as an acoustic ink printer. This printer utilizes a focused acoustic beam formed with a spherical lens illuminated by a plane wave of sound created by a piezoelectric transducer. The focused acoustic beam exerts pressure on the surface of the liquid, resulting in the ejection of small droplets of ink onto an imaging substrate.
Conventional inks for inkjet printers generally comprise a colorant such as dye which is soluble in a vehicle of water or a mixture comprising water and a water-soluble or water-miscible organic solvent. However, dyes have several disadvantages when used in inkjet inks. Dyes, being water-soluble or soluble in a water and organic mixture, may redissolve and run when exposed to moisture or water. Dye images may smear and rub off on contact with felt pen markers or upon being rubbed with a finger. Dyes may also exhibit poor light stability, including fluorescence, sunlight, and ultraviolet light. Inks comprising soluble dyes may also exhibit clogging of the jetting channels due to solvent evaporation and changes in the dye's solubility, dye crystallization, and the presence of impurities. Dye-based ink may also exhibit poor thermal and chemical stability which could result in poor print quality. The dye may also bleed or diffuse into pores or along fibers of the paper, thus causing poor print quality and low optical density of the image.
Pigments have also been used as colorants in inkjet inks, either as substitutes for, or in combination with, dyes. Pigments offer properties superior to dyes in areas such as waterfastness, lightfastness, image density, thermal stability, oxidative and chemical stability, compatibility with other colorants, and compatibility with both coated/treated and plain papers. Pigments used in inkjet inks include carbon black, titanium dioxide, cobalt blue (CoO--Al.sub.2 O.sub.3), chrome yellow (PbCrO.sub.4), phthalocyanine blue, and other organic pigments. Other pigments include inorganic and polymer pigments, wherein these pigments and organic pigments can have dyes absorbed and/or incorporated therein. Carbon black, which absorbs in the infrared region, may be used for bar code reading.
The major problem with the use of such pigments in inkjet systems is initial dispersion and dispersion stability. Pigment particles such as carbon black generally start in a clumped or agglomerated state. To prepare inkjet inks, however, the carbon black must be dispersed and stabilized in that form because the extent of dispersion directly affects inkjet printing characteristics such as ejectability, print quality, optical density, and the like. Additionally, since the nozzle openings of thermal or bubble type inkjet printers are typically about 10-60 micrometers in diameter, it is critical to ensure that the inkjet inks do not clog or plug these openings. Thus, it is necessary to make the pigment particles as small as possible. Preferably carbon black is reduced to individual aggregates. Small pigment particles are also less prone to settling during storage and therefore further contribute to the stability of the carbon black dispersion.
In light of these requirements, conventional aqueous pigment-based inkjet inks generally contain an aqueous ink vehicle, a pigment, a dispersant, and a humectant to prevent drying of ink or the clogging of orifices. Further additives such as biocides, binders, salts, driers, penetrants, surfactants, and the like may also be included.
The conventional inks, whether containing a colorant which is a dye, a pigment or a combination thereof, require improvement in a number of properties. In particular, it is desirable to decrease drying time of the ink, increase water-and-smear-resistance of the images, increase optical density of the print, and provide better quality images with less intercolor bleed.